Four wheel steering system

ABSTRACT

A four wheel steering system for a motor vehicle with steerable front and rear road wheels, a steering angle sensor for detecting a front wheel steering angle through which the front road wheels are steered, a vehicle speed sensor for detecting a vehicle speed of the motor vehicle, a rear wheel steering mechanism for steering the rear wheels, and a controller for controlling the rear wheel steering mechanism to steer the rear road wheels based on the detected front wheel steering angle and the detected vehicle speed when the front road wheels are steering. The controller controls the rear wheel steering mechanism to steer the rear road wheels in a direction which is opposite to the front road wheels if the detected vehicle speed is relatively low, and also controls the rear wheel steering mechanism to steer the rear road wheels in a direction which is the same as the front road wheels if the detected vehicle speed is relatively high. Furthermore, the controller controls the rear wheel steering mechanism to increase a rear wheel steering angle which through the rear road wheels are steered as the front wheel steering angle increases if a change in the front wheel steering angle is relatively small, and controls the rear wheel steering mechanism to reduce an increase in the rear wheel steering angle if a change in the front wheel steering angle is relatively large.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a four wheel steering system for motor vehicles, and more particularly to a four wheel steering system for steering rear road wheels of a motor vehicle depending on a change in the steering angle of front road wheels of the motor vehicle.

2. Description of the Prior Art

One conventional four wheel steering system for motor vehicles is shown in Japanese Patent Publication No. 1-12714, for example. The disclosed four wheel steering system steers the rear road wheels in the opposite direction to the front road wheels when the driver's steering action is quick, and steers the rear road wheels in the same direction as the front road wheels when the driver's steering action is slow.

However, the conventional four wheel steering system is disadvantageous in that if the driver attempts to quickly steer the motor vehicle when the rear road wheels have already been steered in the opposite direction to the front road wheels by a quick steering action, then the rear road wheels are steered through a large steering angle, adversely affecting the behavior of the motor vehicle.

SUMMARY OF THE INVENTION

In view of the aforesaid problem of the prior four wheel steering system, it is an object of the present invention to provide a four wheel steering system which allows a motor vehicle to be steered with stable behaviors at all times irrespective of whether the driver's action to steer the front wheels is quick or slow.

According to the present invention, there is provided a four wheel steering system for a motor vehicle with steerable front and rear road wheels, comprising means for detecting a front wheel steering angle through which the front road wheels are steered, means for detecting a vehicle speed of the motor vehicle, a mechanism for steering the rear wheels, and control means for controlling the mechanism to steer the rear road wheels based on the detected front wheel steering angle and the detected vehicle speed when the front road wheels are steered, the control means comprising means for controlling the mechanism to steer the rear road wheels in a direction which is opposite to the front road wheels if the detected vehicle speed is relatively low, controlling the mechanism to steer the rear road wheels in a direction which is the same as the front road wheels if the detected vehicle speed is relatively high, controlling the mechanism to increase a rear wheel steering angle through which the rear road wheels are steered as the front wheel steering angle increases if a change in the front wheel steering angle is relatively small, and controlling the mechanism to reduce an increase in the rear wheel steering angle if a change in the front wheel steering angle is relatively large.

The above and further objects, details and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof, when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of a four wheel steering system, as incorporated in a motor vehicle, according to the present invention;

FIG. 2 is a flowchart of a control process according to a first embodiment of the present invention, which is to be executed by a controller in the four wheel steering system;

FIG. 3 is a data table 1 of vehicle speeds V and steering angle ratios k, used in the control process shown in FIG. 2;

FIG. 4 is a data table 2 of front wheel steering angles θ_(F) and corrective coefficients α, used in the control process shown in FIG. 2;

FIG. 5 is a data table 3 of front wheel steering angle differences Δθ_(F) and corrective steering angle changes θ_(f), used in the control process shown in FIG. 2;

FIG. 6 is a flowchart of a control process according to a second embodiment of the present invention, which is to be executed by the controller in the four wheel steering system; and

FIG. 7 a data table 4 of front wheel steering angular velocity differences Δθ_(F) and corrective coefficients β, used in the control process shown in FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically shows a four wheel steering system, as incorporated in a motor vehicle, according to the present invention.

As shown in FIG. 1, the motor vehicle has a steering wheel 11 operatively coupled through a steering shaft 12 to a rack-and-pinion front wheel steering gear mechanism in a front gear housing 13F mounted on a front portion of the body of the motor vehicle. The steering shaft 12 is rotatably disposed in a column 18 that is associated with a steering angle sensor 14 and a steering speed sensor 15. The steering angle sensor 14 comprises an encoder or the like for detecting the angle through which the steering shaft 12 rotates about its own axis. The steering speed sensor 15 comprises a tachometer generator or the like for detecting the angular velocity at which the steering shaft 12 rotates about its own axis. The sensors 14, 15 send detected signals to a controller 20 electrically connected thereto.

The steering gear mechanism comprises a pinion (not shown) rotatable with the steering shaft 12 and a rack (not shown) extending transversely of the motor vehicle. The rack has opposite ends coupled respectively to left and right front road wheels 17FL, 17FR through respective steering linkages such as tie rods 16FL, 16FR, for transmitting a driver's steering action from the steering wheel 11 to the front road wheels 17FL, 17FR. The front road wheels 17FL, 17FR and rear road wheels 17RL, 17RR are associated with respective vehicle speed sensors 19FL, 19FR, 19RL, 19RR (which will also be generally referred to as speed sensors 19). These speed sensors 19 are electrically connected to the controller 20. Instead of the sensors 14, 15, there may be employed sensors for detecting a steering angle and a steering angular velocity of the front road wheels 17FL, 17FR directly from the front road wheels 17FL, 17FR.

A rear gear housing 13R is mounted on a rear portion of the motor vehicle body, and houses a rear wheel steering mechanism (not shown). The rear wheel steering mechanism comprises a rod slidably supported in the housing 13R and extending transversely of the motor vehicle, an electric motor for axially moving the rod, and a rear wheel steering angle sensor 24 for detecting the axial displacement of the rod. The rod of the rear wheel steering mechanism has opposite ends connected respectively to the rear road wheels 17RL, 17RR through respective tie rods 16RL, 16RR. The electric motor and the rear wheel steering angle sensor 24 are electrically connected to the controller 20. The electric motor is energized by the controller 20 to steer the rear road wheels 17RL, 17RR. When the rod of the rear wheel steering mechanism is axially displaced, the rear wheel steering angle sensor 24 detects the corresponding steering angle of the rear road wheels 17RL, 17RR, and applies a detected signal to the controller 20.

The controller 20 comprises a microcomputer connected to the sensors 14, 15, 19, 24 and a motor driver (not shown) connected to the electric motor of the rear wheel steering mechanism 13R. The microcomputer processes output signals from the sensors 14, 15, 19, 24 to determine a target steering angle and a target steering angular velocity for the rear road wheels 17RL, 17RR, determines a direction in which an electric current is to be supplied to the electric motor and a duty factor with which the electric motor is to be energized, and outputs a PWM signal indicative of the determined direction and duty factor to the motor driver. The motor driver comprises a bridge of FETs or the like for energizing the electric motor based on the PWM signal from the microcomputer.

Operation of the four wheel steering system shown in FIG. 1 will be described below with reference to FIG. 2.

The microcomputer of the controller 20 executes the control process shown in FIG. 2 to control the electric motor in the rear gear housing 13R, i.e., to control the steering operation of the rear road wheels 17RL, 17RR.

First, when the driver of the motor vehicle turns the ignition key to shift the key switch to the ON position, the controller 20 is energized by the battery of the motor vehicle, starting to operate the microcomputer. The microcomputer reads a vehicle speed V from the output signals of the vehicle speed sensors 19 in a step P₁. Then, in a step P₂, the microcomputer looks for a steering angle ratio k addressed by the vehicle speed V in a data table 1 shown in FIG. 3 that is stored in a ROM of the microcomputer. The steering angle ratio is a ratio between the steering angle of the front road wheels and the steering angle of the rear road wheels. The data table 1 contains reference steering angle ratios k with respect to corresponding vehicle speeds. As shown in FIG. 3, the steering angle ratio k is zero at a predetermined vehicle speed V₀, negative in a vehicle speed range lower than the vehicle speed V₀ (V<V₀), and positive in a vehicle speed range higher than the vehicle speed V₀ (V>V₀). The positive steering angle ratios k indicate that the front and rear road wheels are in the same phase, meaning that the front and rear road wheels are steered in the same direction, and the negative steering angle ratios k indicate the front and rear road wheels are in the opposite phase, meaning that the front and rear road wheels are steered in the opposite directions.

Then, the microcomputer reads a front wheel steering angle θ_(F) from the output signal of the steering angle sensor 14 in a step P₃. The microcomputer determines whether the vehicle speed V read in the step P1 exceeds the predetermined vehicle speed V₀ or not in a step P₄. Since the predetermined vehicle speed V₀ is the same as the vehicle speed V₀ used in the step P₂, the step P₄ is equivalent to determining whether the steering angle ratio is positive or negative, i.e., whether the front and rear road wheels are in the same phase or in the opposite phase. If the vehicle speed V is equal to or lower than the predetermined vehicle speed V₀, then the microcomputer multiplies the front wheel steering angle θ_(F) by the steering angle ratio k, producing a target rear wheel steering angle θ_(RT) in a step P₉.

If the vehicle speed V is higher than the predetermined vehicle speed V₀, then control goes from the step P₄ to a step P₅ in which the microcomputer searches the data table 2 for a corrective coefficient α addressed by the front wheel steering angle θ_(F). As shown in FIG. 4, the corrective coefficient α is of a constant value of 1 if the front wheel steering angle θ_(F) is smaller than a predetermined front wheel steering angle b, and is progressively reduced if the front wheel steering angle θ_(F) goes larger than the predetermined front wheel steering angle b. This means that the motor vehicle can be steered for easy smaller turns when the front wheel steering angle θ_(F) is relatively large while the front and rear road wheels are in the same phase.

In a next step P₆, the microcomputer subtracts a previous front wheel steering angle θ_(F0) read in a previous routine of the control process from the front wheel steering angle θ_(F) read in the step P₃ of the present routine of the control process, thus producing a front wheel steering angle difference Δθ_(F). The front wheel steering angle difference Δθ_(F) may be regarded as being equivalent to a front wheel steering angular velocity θ_(F) since the control process is carried out at predetermined cyclic periods. Then, in a step P₇, the microcomputer determines a corrective steering angle change θ_(f) addressed by the front wheel steering angle difference Δθ_(F) from a data table 3 shown in FIG. 5. In FIG. 5, if the front wheel steering angle difference Δθ_(F) is smaller than a predetermined value c, then the corrective steering angle change θ_(f) is of a positive value and given as a linear function of the front wheel steering angle difference Δθ_(F). If the front wheel steering angle difference Δθ_(F) is larger than the predetermined value c, then the corrective steering angle change θ_(f) is of a positive value and given as a parabolic function, which is upwardly convex, of the front wheel steering angle difference Δθ_(F). Therefore, the corrective steering angle change θ_(f) becomes progressively smaller as the front wheel steering angle difference Δθ_(F) is increased in a relatively large range thereof. In a step P₈, the microprocessor multiplies the sum of the previous front wheel steering angle θ_(F0) and the corrective steering angle change θ_(f) by the corrective coefficient α and the steering angle ratio k, thus determining a target rear wheel steering angle θ_(RT). If the corrective steering angle change θ_(f) is small, then the difference between the target rear wheel steering angle θ_(RT) thus determined and the target rear wheel steering angle θ_(RT) in the previous routine is small. For example, since the corrective steering angle change θ_(f) is small when the front wheel steering angle difference Δθ_(F) is large, the difference between the target rear wheel steering angle θ_(RT) thus determined and the target rear wheel steering angle θ_(RT) in the previous routine may be smaller than it is when the front wheel steering angle difference Δθ_(F) is small.

More specifically, the target rear wheel steering angle θ_(RT) is expressed by:

    θ.sub.RT =k×α×(θ.sub.f0 +θ.sub.f) (1)

If the front wheel steering angle difference Δθ_(F) is smaller than the value c, then the corrective steering angle change θ_(f) is given as:

    θ.sub.f =θ.sub.F -θ.sub.F0               (2)

(assuming that the linear portion of the corrective steering angle change θ_(f) below the value c has a proportional coefficient of 1), and the equation (1) becomes:

    θ.sub.RT =k×α×θ.sub.F        (1)'

The equation (1)' is a function which is not related to the previous front wheel steering angle θ_(F0) and is not affected by the steering angle change. Therefore, when the steering action is slow, the target rear wheel steering angle θ_(RT) is determined by the vehicle speed and the steering angle at the time.

When the steering speed is increased and the front wheel steering angle difference Δθ_(F) exceeds the value c, the corrective steering angle change θ_(f) becomes smaller than the front wheel steering angle difference Δθ_(F), which is used as the value of θ_(f) for determining the target rear wheel steering angle θ_(RT) as expressed by the equation (1)' because the corrective steering angle change θ_(f) is represented by an upwardly convex curve and is smaller than the proportional function indicated by the dotted line in FIG. 5. As the front wheel steering angle difference Δθ_(F) is larger, since θ_(f) ≈0, the equation (1) becomes:

    θ.sub.RT ≈k×α×θ.sub.F0 (1)"

When the steering action is quick, the increase in the front wheel steering angle difference Δθ_(F) is reduced. However, insofar as the vehicle speed is constant, the target rear wheel steering angle θ_(RT) does not become smaller than the rear wheel steering angle which has been achieved according to the previous front wheel steering angle θ_(F0) when the front and rear road wheels are steered in the same phase, no matter how quick the steering action may be, provided the front wheel steering angle is in a range below the value b as when the motor vehicle runs at high speed. Even when the front wheel steering angle is greater than the value b, any reduction in the rear wheel steering angle due to the corrective coefficient α when the front and rear road wheels are steered in the same phase is held to a minimum.

In a next step P₁₀, the microcomputer reads a rear wheel steering angle θ_(R) from the output signal of the rear wheel steering angle sensor 24. Then, the microcomputer calculates the difference Δθ_(R) between the target rear wheel steering angle θ_(RT) and the read rear wheel steering angle θ_(R) in a step P₁₁. In a step P₁₂, the microcomputer controls the motor driver to energize the electric motor in the gear housing 13R for thereby turning or steering the rear road wheels 17RL, 17RR to the target rear wheel steering angle θ_(RT). Then, the microcomputer stores the front wheel steering angle θ_(F) in preparation for a next routine in a step P₁₃. Then, the microcomputer repeats the control process from the step P₁ all over again.

With the four wheel steering system according to the above embodiment, as described above, the target rear wheel steering angle θ_(RT) is corrected depending on the front wheel steering angle difference Δθ_(F) between the front wheel steering angle θ_(F0) in the previous routine and the front wheel steering angle θ_(F) in the present routine, i.e., depending on the steering speed, such that the change in the target rear wheel steering angle θ_(RT), which tends to increase when the front wheel steering angle difference Δθ_(F) is larger, is reduced. Therefore, even when a quick steering action is additionally effected from the condition in which the rear road wheels 17RL, 17RR have been steered in the same phase as the front road wheels 17FL, 17FR, the direction in which the rear road wheels 17RL, 17RR are steered does not vary, and the change in the steering angle of the rear road wheels 17RL, 17RR is reduced. Consequently, the four wheel steering system keeps the motor vehicle highly stable while increasing a yaw response which is a response of the motor vehicle to the steering action in making a turn.

FIG. 6 shows a control process according to a second embodiment of the present invention, and FIG. 7 shows a data table used in the control process shown in FIG. 6. The control process shown in FIG. 6 can be executed by the controller 20 and used in combination with the motor vehicle shown in FIG. 1. The control process shown in FIG. 6 contains steps P₁ through P₄ and P₁₀ through P₁₄ that are identical to the steps P₁ through P₄ and P₉ through P₁₃, and hence these steps will not be described below.

In a step P₅, the microcomputer reads a front wheel steering angular velocity θ_(F) from the output signal of the steering speed sensor 15. Then, the microcomputer searches the data table 2 shown in FIG. 4 for a corrective coefficient α addressed by the front wheel steering angle θ_(F) in a step P₆. In a next step P₇, the microcomputer subtracts a previous front wheel steering angular velocity θ_(F0) read in a previous routine from the front wheel steering angular velocity θ_(F) read in the step P₅ of the present routine, thus producing a front wheel steering angular velocity difference Δθ_(F). The front wheel steering angular velocity difference Δθ_(F) may be regarded as being equivalent to a front wheel steering angular acceleration θ_(F) since the control process is carried out at predetermined cyclic periods. Then, in a step P₈, the microcomputer determines a corrective coefficient β addressed by the front wheel steering angular velocity difference Δθ_(F) from a data table 4 shown in FIG. 7. In FIG. 7, the corrective coefficient β is a linear function of the front wheel steering angular velocity difference Δθ_(F) and is linearly proportional thereto with a positive proportional constant.

In a next step P₉, the microcomputer calculates a target rear wheel steering angle θ_(RT) according to the following equation (3):

    θ.sub.RT =k×α×}(1-β)θ.sub.F +β×θ.sub.F0 .tbd.                        (3)

When the steering acceleration is small, since β≈0, as can be seen from the equation (3) and FIG. 7, the equation (3) becomes:

    θ.sub.RT =k×α×θ.sub.F        (3)'

and hence the target rear wheel steering angle θ_(RT) is determined by the front wheel steering angle and the vehicle speed while the front and rear road wheels are being steered in the same phase.

When the steering acceleration is large, the corrective coefficient β=1, and the equation (3) becomes:

    θ.sub.RT =k×α×θ.sub.F0       (3)"

Thus, it can be understood that when the steering action is effected with a high steering acceleration, the target rear wheel steering angle θ_(RT) is reduced while the front and rear road wheels are being steered in the same phase.

In the second embodiment, a higher yaw response is attained when the steering wheel is additionally turned quickly while the motor vehicle is making a turn. When the steering action is finished, the target rear wheel steering angle θ_(RT) quickly returns to the angle that is given according to the front wheel steering angle, making the motor vehicle highly stable.

The corrective processes using the steering angular velocity and the steering acceleration may be carried out simultaneously.

While the steering operation of the rear road wheels is controlled based on the front wheel steering angle θ_(F), the front wheel steering angular velocity θ_(F), and the front wheel steering angular acceleration θ_(F) in the above embodiments, since the front wheel steering angle θ_(F) may be regarded as the angle through which the steering wheel 11 is turned, the steering operation of the rear road wheels may be controlled based on the angle through which the steering wheel 11 is turned, and other variables related to the steering wheel 11.

Since the change in the rear wheel steering angle is reduced when the steering action is effected at high steering speed, the four wheel steering system according to the present invention keeps the motor vehicle stable while increasing the yaw response thereof.

Although there have been described what are at present considered to be the preferred embodiments of the invention, it will be understood that the invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. 

We claim:
 1. A four wheel steering system for a motor vehicle with steerable front and rear road wheels, comprising:means for detecting a front wheel steering angle θ_(F) through which the front road wheels are steered; means for detecting a vehicle speed V of the motor vehicle; means for determining a change in the front wheel steering angle; means for steering the rear road wheels; and control means for controlling said rear road wheels steering means based on the detected front wheel steering angle and the detected vehicle speed when the front road wheels are steered, said control means comprising means for controlling said steering means to steer the rear road wheels in a direction which is opposite to the front road wheels if the detected vehicle speed is smaller than a predetermined vehicle speed, controlling said steering means to steer the rear road wheels in a direction which is the same as the front road wheels if the detected vehicle speed is larger than the predetermined vehicle speed, controlling said steering means to increase a rear wheel steering angle through which the rear road wheels are steered as the front wheel steering angle increases if a change in the front wheel steering angle is smaller than a predetermined change in the front wheel steering angle, and controlling said steering means to reduce an increase in the rear wheel steering angle if a change in the front wheel steering angle is larger than said predetermined change in the front wheel steering angle.
 2. A four wheel steering system according to claim 1, wherein said control means comprises a microcomputer for repeatedly executing a control process at cyclic periods, and controlling said steering means to steer the rear road wheels to a target rear wheel steering angle θ_(RT), said control process including an arithmetic operation to determine said target rear wheel steering angle θ_(RT) as defined by the following equation:

    θ.sub.RT =k×α×(θ.sub.F0 +O.sub.f)

where k is a ratio of the front wheel steering angle to the rear wheel steering angle, the ratio k being of a positive value when the vehicle speed V is higher than a predetermined vehicle speed V₀ (V>V₀) and at which the rear road wheels are steered in the same direction as the front road wheels, α is a corrective coefficient which is constant when the front wheel steering angle θ_(F) is smaller than a predetermined angle b and is progressively smaller when the front wheel steering angle θ_(F) is larger than said predetermined angle b, θ_(F0) is a front wheel steering angle detected in a previous routine of the control process, and θ_(f) is a corrective steering angle change corresponding to a front wheel steering angle difference Δθ_(F) between the front wheel steering angle Δ_(F0) detected in the previous routine and the front wheel steering angle θ_(F) detected in a present routine of the control process.
 3. A four wheel steering system according to claim 2, wherein said corrective steering angle change θ_(f) is represented by a characteristic curve which is of a linearly varying positive value when said front wheel steering angle difference Δθ_(F) is smaller than a predetermined value c, and which is of an upwardly convex, parabolic positive value when said front wheel steering angle difference Δθ_(F) is larger than said predetermined value c.
 4. A four wheel steering system according to claim 3, wherein said control means comprises means for controlling said steering means to steer the rear road wheels, based on only the vehicle speed and the front wheel steering angle, to a target rear wheel steering angle θ_(RT) as defined by the following equation:

    θ.sub.RT =k×α×θ.sub.F

when said front wheel steering angle difference Δθ_(F) is smaller than said predetermined value c with θ_(f) ≈θ_(F) -θ_(F0).
 5. A four wheel steering system according to claim 3, wherein said control means comprises means for controlling said steering means to steer the rear road wheels to a target rear wheel steering angle θ_(RT) as defined by the following equation:

    θ.sub.RT =k×α×θ.sub.F0

when said front wheel steering angle difference Δθ_(F) is larger than said predetermined value c with θ_(f) =0, whereby a change in the rear wheel steering angle is reduced when the front wheels are additionally steered from a condition in which the front and rear road wheels are already being steered in the same direction.
 6. A four wheel steering system according to claim 2, wherein said ratio k is zero when said vehicle speed V is equal to said predetermined vehicle speed V₀, and is of a negative value when the vehicle speed V is lower than said predetermined vehicle speed V₀ (V<V₀) and at which the rear road wheels are steered in the opposite direction to the front road wheels, and wherein said control process further includes an arithmetic operation to determine said target rear wheel steering angle θ_(RT) as defined by the following second equation:

    θ.sub.RT =k×θ.sub.F,

said control means comprising means for controlling said steering means to steer the rear road wheels to the target rear wheel steering angle θ_(RT) according to said second equation when the detected vehicle speed V is at most said predetermined vehicle speed V₀ (V≦V₀).
 7. A four wheel steering system according to claim 1, further including means for detecting a front wheel steering angular velocity θ_(F) at which the front road wheels are steered, and wherein said control means comprises a microcomputer for repeatedly executing a control process at cyclic periods, and controlling said steering means to steer the rear road wheels to a target rear wheel steering angle θ_(RT), said control process including an arithmetic operation to determine said target rear wheel steering angle θ_(RT) as defined by the following equation:

    θ.sub.RT =k×α×}(1-β)θ.sub.F +β×θ.sub.F0 }

where k is a ratio of the front wheel steering angle to the rear wheel steering angle, the ratio k being of a positive value when the vehicle speed V is higher than a predetermined vehicle speed V₀ (V>V₀) and at which the rear road wheels are steered in the same direction as the front road wheels, α is a corrective coefficient which is constant when the front wheel steering angle θ_(F) is smaller than a predetermined angle b and is progressively smaller when the front wheel steering angle θ_(F) is larger than said predetermined angle b, θ_(F0) is a front wheel steering angle detected in a previous routine of the control process, and β is a corrective coefficient corresponding to a front wheel steering angular velocity difference Δθ_(F) between the front wheel steering angular velocity θ_(F0) detected in the previous routine and the front wheel steering angular velocity θ_(F) detected in a present routine of the control process.
 8. A four wheel steering system according to claim 7, wherein said corrective coefficient β is represented by a linear function which is linearly proportional to said front wheel steering angular velocity difference Δθ_(F) with a positive proportional constant.
 9. A four wheel steering system according to claim 8, wherein said control means comprises means for controlling said steering means to steer the rear road wheels, based on only the vehicle speed and the front wheel steering angle, to a target rear wheel steering angle θ_(RT) as defined by the following equation:

    θ.sub.RT =k×α×θ.sub.F

when said front wheel steering angular velocity difference Δθ_(F) is relatively small with β≈0.
 10. A four wheel steering system according to claim 7, wherein said control means comprises means for controlling said steering means to steer the rear road wheels to a target rear wheel steering angle θ_(RT) as defined by the following equation:

    θ.sub.RT =k×α×θ.sub.F0

when said front wheel steering angular velocity difference Δθ_(F) is relatively large with β≈1, whereby a change in the rear wheel steering angle is reduced when the front wheels are additionally steered from a condition in which the front and rear road wheels are already being steered in the same direction.
 11. A four wheel steering system according to claim 7, wherein said ratio k is zero when said vehicle speed V is equal to said predetermined vehicle speed V_(O), and is of a negative value when the vehicle speed V is lower than said predetermined vehicle speed V₀ (V<V_(O)) and at which the rear road wheels are steered in the opposite direction to the front road wheels, and wherein said control process further includes an arithmetic operation to determine said target rear wheel steering angle θ_(RT) as defined by the following second equation:

    θ.sub.RT =k×θ.sub.F,

said control means comprising means for controlling said steering means to steer the rear road wheels to the target rear wheel steering angle θ_(RT) according to said second equation when the detected vehicle speed V is at most said predetermined vehicle speed V₀ (V<V₀).
 12. A four wheel steering system for a motor vehicle with steerable front and rear road wheels, comprising:means for detecting a front wheel steering angle θ_(F) through which the front road wheels are steered, said steering angle detecting means detecting the front wheel steering angle at predetermined cyclical intervals; means for detecting a vehicle speed V of the motor vehicle; means for determining a change in the front wheel steering angle as a difference between a most recently detected front wheel steering angle and a next most recently detected front wheel steering angle; means for steering the rear wheels; and control means for controlling said steering means based on the detected front wheel steering angle and the detected vehicle speed when the front road wheels are steered, said control means comprising means for controlling said steering means to steer the rear road wheels in a direction which is opposite to the front road wheels if the detected vehicle speed is smaller than a predetermined vehicle speed, controlling said steering means to steer the rear road wheels in a direction which is the same as the front road wheels if the detected vehicle speed is larger than the predetermined vehicle speed, controlling said steering means to increase a rear wheel steering angle through which the rear road wheels are steered in proportion to a determined increase in the detected change in the front wheel steering angle if the detected change is smaller than a predetermined change, and controlling said steering means to increase the rear wheel steering angle through which the rear road wheels are steered to a reduced extent not in proportion to the detected change in the front wheel steering angle if the detected change is larger than said predetermined change.
 13. A four wheel steering system for a motor vehicle with steerable front and rear road wheels, comprising:means for detecting a front wheel steering angle θ_(F) through which the front road wheels are steered; means for detecting a front wheel steering angular velocity θ_(F) at which the front road wheels are steered, said steering angular velocity detecting means detecting said steering angular velocity at predetermined cyclic intervals; means for detecting a vehicle speed V of the motor vehicle; means for determining a change in a front wheel steering angular velocity based on a difference between a most recently detected front wheel steering angular velocity and a next most recently detected front wheel steering angular velocity; means for steering the rear wheels; and control means for controlling said steering means based on the detected front wheel steering angle, the detected steering angular velocity and the detected vehicle speed when the front road wheels are steered, said control means comprising means for controlling said steering means to steer the rear road wheels in a direction which is opposite to the front road wheels if the detected vehicle speed is smaller than a predetermined vehicle speed, controlling said steering means to steer the rear wheels in a direction which is the same as the front road wheels if the detected vehicle speed is larger than the predetermined vehicle speed, controlling said steering means to increase a rear wheel steering angle through which the rear road wheels are steered in proportion to a determined increase in the front wheel steering angular velocity if the detected change is smaller than a predetermined change, and controlling said steering means to increase the rear wheel steering angle through which the rear road wheels are steered to a reduced extent not in proportion to the detected change in the front wheel steering angular velocity if the detected change is larger than said predetermined change. 